A wide variety of implantable orthopedic implants and procedures are known for stabilizing and securing fractures in bones, replacing damaged joints, attaching tissue to bone, and the like. For example, fixation plates and intramedullary devices can be surgically positioned to span the fracture site. Intramedullary devices are also commonly used to attach replacement joints to long bones. A variety of orthopedic fasteners, such as screws, pins, and the like, are typically used to help secure these orthopedic implants to the bone.
The ability of orthopedic fasteners to resist loosening is related to bone quality (O. R. Zindric et al Clinical Orthopaedics (1986) 203:99-112), while the holding power of an orthopedic fastener correlates with mineral density (T. C. Ryken et al Journal of Neurosurgery (1995) 83:325-329). If the bone at the implantation site is compromised, either before, such as due to osteoporosis, or as a result of the implantation procedure, the surgeon may have limited options for securing the orthopedic implant.
Loosening and backing out of an orthopedic fasteners can result in decreased structural integrity of the bone. Once an orthopedic fastener manages to work itself loose, wear and tear to the opening or space in the bone within which it was received may prohibit securely refastening the orthopedic fastener in the bone. Adding more orthopedic fasteners to compensate for the compromised bone complicates future revision or removal, and may further weaken the bone. For example, the formation of screw holes in the cortical bone provides stress risers that substantially increases the risk of bone re-fracture. Since orthopedic implants interfere with revascularization in the bone it is preferred to minimize the number of such devices.
U.S. Pat. Nos. 7,789,901 and 8,241,340 (Froehlich) discloses an expandable structure fixedly attached to a distal end of a bone anchor. The expandable structure is configured to expand when a filler material is delivered through a fill port and into the expandable structure. The distal end of the bone anchor is embedded in the cured filler material to form a permanent connection with the bone.
U.S. Pat. No. 7,717,947 (Wilberg et al.) discloses a cannulated bone screw with an axial bore and exit ports near the threads. Bone cement is injected through the axial bore and flows out the exit ports to permanently anchor the bone screw in the bone. The bone cement is located at the interface of the bone screw to the bone.
U.S. Pat. No. 7,488,320 (Middleton) discloses an anchor for an orthopedic implant similar to Wilberg with lumens for injecting bone cement. The bone cement forms an interlocking relationship with structures and voids on a preformed element to permanently anchor the device in the bone. Once the injectable material is hardened, the anchors of Wilberg is permanently locked in position.
The strategies noted above rely on bone cement to augment pull out strength. PMMA is exothermic upon polymerization and toxic monomers can cause bone necrosis, proliferation of fibrous tissue layers and other adverse biological responses (H. C. M. Amstutz et al Clin. Orthop. (1992) 276:7-18 and J. G. Heller et al J. Bone J. Surg. [Am] (1996) 78:1315-1321). Cement induced osteolysis or necrotic bone may impair the fixation and lead to eventual fastener loosening and failure. In the case of failure it is often difficult to remove cement from the bone and it is usually associated with excessive damage to the surrounding bone.
In some cases an orthopedic implant may need to be adjusted or corrected after the original implantation surgery is completed. Such revisions may be necessitated by re-fracture, infection, deterioration of the bone, situations where the patient's subsequent growth requires revision of the implant so as not to impede proper growth, and the need to move corrective forces of the orthopedic implant on an area or in an orientation that is different from what was originally needed. In those cases, an adjustment, correction or other revision of the implanted orthopedic implant will require unlocking and removal of the orthopedic fasteners. Bone cement at the interface with the orthopedic fasteners greatly complicates this procedure.
A number of cementless solutions have been proposed, such as interlocking screws (B. E. McKoy, 47.sup.th Annual Meeting, Orthopaedic Research Society, Feb. 25-28, 2001, Session 19, Bone Mechanics II) and bone screw anchors (B. E. McKoy and Y. H. An Journal of Orthopaedic Research (2001) 19:545-547). Other bone implantation/fixation devices and methods are known in the art, for example, U.S. 2004/0181225, U.S. Pat. No. 5,084,050, U.S. Pat. No. 5,720,753, U.S. Pat. No. 6,656,184, U.S. Pat. No. 6,517,542, and U.S. Pat. No. 6,835,206. Helical anchors are generally well known, for example, U.S. Pat. No. 806,406, U.S. Pat. No. 3,983,736, U.S. Pat. No. 4,536,115, U.S. Pat. No. 5,312,214, U.S. Pat. No. 6,276,883, U.S. Pat. No. 6,494,657, and U.S. Pat. No. 6,860,691. Furthermore, helically wound springs have been described for use as tissue anchors (WO 01/08602) and helical coils have been described for use as surgical implants (U.S. 2004/0225361).